Practical scada for industry 1 110

Practical SCADA
for Industry


Titles in the series
Practical Cleanrooms: Technologies and Facilities (David Conway)
Practical Data Acquisition for Instrumentation and Control Systems (John Park,
Steve Mackay)
Practical Data Communications for Instrumentation and Control (John Park, Steve
Mackay, Edwin Wright)
Practical Digital Signal Processing for Engineers and Technicians (Edmund Lai)
Practical Electrical Network Automation and Communication Systems (Cobus
Strauss)
Practical Embedded Controllers (John Park)
Practical Fiber Optics (David Bailey, Edwin Wright)
Practical Industrial Data Networks: Design, Installation and Troubleshooting (Steve
Mackay, Edwin Wright, John Park, Deon Reynders)
Practical Industrial Safety, Risk Assessment and Shutdown Systems (Dave
Macdonald)
Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems (Gordon
Clarke, Deon Reynders)
Practical Radio Engineering and Telemetry for Industry (David Bailey)
Practical SCADA for Industry (David Bailey, Edwin Wright)
Practical TCP/IP and Ethernet Networking (Deon Reynders, Edwin Wright)
Practical Variable Speed Drives and Power Electronics (Malcolm Barnes)


Practical SCADA
for Industry
David Bailey BEng, Bailey and Associates, Perth, Australia
+J]OT=XOMNZ MIPENZ, BSc(Hons), BSc(Elec Eng), IDC Technologies, Perth,

Australia


Newnes
An imprint of Elsevier
Linacre House, Jordan Hill, Oxford OX2 8DP
200 Wheeler Road, Burlington, MA 01803
First published 2003
Copyright  2003, IDC Technologies. All rights reserved
No part of this publication may be reproduced in any material form (including
photocopying or storing in any medium by electronic means and whether
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the written permission of the copyright holder except in accordance with the
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a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road,
London, England W1T 4LP. Applications for the copyright holder's written
permission to reproduce any part of this publication should be addressed
to the publisher

British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library

ISBN 07506 58053
For information on all Newnes publications, visit
our website at www.newnespress.com

Typeset and Edited by Vivek Mehra, Mumbai, India
()
Printed and bound in Great Britain

Contents
Preface
1
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9

2
2.1
2.2

2.3

2.4
2.5

2.6

xiii
Background to SCADA

1

Introduction and brief history of SCADA
Fundamental principles of modern SCADA systems
SCADA hardware
SCADA software
Landlines for SCADA
SCADA and local area networks
Modem use in SCADA systems
Computer sites and troubleshooting
System implementation

1
2
4
5
6
7
7
8
9

SCADA systems, hardware and firmware

11

Introduction
Comparison of the terms SCADA, DCS, PLC and smart instrument

11
12

2.2.1
2.2.2
2.2.3
2.2.4
2.2.5

12
15
15
16
17

SCADA system
Distributed control system (DCS)
Programmable logic controller (PLC)
Smart instrument
Considerations and benefits of SCADA system

Remote terminal units

17

2.3.1
2.3.2
2.3.3
2.3.4
2.3.5
2.3.6
2.3.7
2.3.8

2.3.9
2.3.10
2.3.11
2.3.12
2.3.13

19
19
26
27
28
29
31
33
33
33
33
34
35

Control processor (or CPU)
Analog input modules
Typical analog input modules
Analog outputs
Digital inputs
Counter or accumulator digital inputs
Digital output module
Mixed analog and digital modules
Communication interfaces
Power supply module for RTU

RTU environmental enclosures
Testing and maintenance
Typical requirements for an RTU system

Application programs
PLCs used as RTUs

36
36

2.5.1
2.5.2
2.5.3

37
38
40

PLC software
Basic rules of ladder-logic
The different ladder-logic instructions

The master station

46

2.6.1

48

Master station software


vi Contents

2.6.2
2.6.3
2.6.4
2.6.5
2.6.6

2.7
2.8

2.9

3

System SCADA software
Local area networks
Ethernet
Token ring LANs
Token bus network

48
48
49
51
52

System reliability and availability

52

2.7.1

52

Redundant master station configuration

Communication architectures and philosophies

54

2.8.1
2.8.2
2.8.3
2.8.4

54
56
56
59

Communication architectures
Communication philosophies
Polled (or master slave)
CSMA/CD system (peer-to-peer)

Typical considerations in configuration of a master station

SCADA systems software and protocols

61

64

3.1
3.2

Introduction
The components of a SCADA system

3.3

The SCADA software package

67

3.3.1
3.3.2
3.3.3

70
72
72

3.2.1

3.4

3.5

3.6

SCADA key features
Redundancy
System response time
Expandability of the system

64
64
65

Specialized SCADA protocols

72

3.4.1
3.4.2
3.4.3
3.4.4
3.4.5

73
74
78
80
81

Introduction to protocols
Information transfer
High level data link control (HDLC) protocol
The CSMA/CD protocol format
Standards activities

Error detection

82

3.5.1
3.5.2

83
84

Causes of errors
Feedback error control

Distributed network protocol

87

3.6.1
3.6.2
3.6.3
3.6.4
3.6.5
3.6.6
3.6.7

3.6.8
3.6.9
3.6.10
3.6.11
3.6.12

87
87
87
88
88
88
88
88
89
92
96
97

Introduction
Interoperability
Open standard
IEC and IEEE
SCADA
Development
Physical layer
Physical topologies
Modes
Datalink layer
Transport layer (pseudo-transport)

Application layer


Contents vii

3.6.13

3.7

3.8

4
4.1
4.2
4.3
4.4

4.5

4.6

4.7

4.8
4.9

Conclusion

97

New technologies in SCADA systems

97

3.7.1
3.7.2
3.7.3
3.7.4

97
97
98
98

Rapid improvement in LAN technology for master stations
Man machine interface
Remote terminal units
Communications

The twelve golden rules

Landlines

98

100

Introduction
Background to cables
Definition of interference and noise on cables

Sources of interference and noise on cables

100
100
101
102

4.4.1
4.4.2
4.4.3

103
104
105

Electrostatic coupling
Magnetic coupling
Impedance coupling

Practical methods of reducing noise and interference on cables

107

4.5.1
4.5.2
4.5.3
4.5.4
4.5.5

107

108
110
111
111

Shielding and twisting wires
Cable spacing
Tray spacing
Earthing and grounding requirements
Specific areas to focus on

Types of cables

112

4.6.1
4.6.2
4.6.3
4.6.4
4.6.5
4.6.6
4.6.7
4.6.8
4.6.9
4.6.10

112
114
114
116

116
116
118
120
120
121

General cable characteristics
Two wire open lines
Twisted pair cables
Coaxial cables
Fiber optics
Theory of operation
Modes of propagation
Specification of cables
Joining cables
Limitations of cables

Privately owned cables

121

4.7.1
4.7.2
4.7.3
4.7.4
4.7.5

121
122

122
122
125

Telephone quality cables
Data quality twisted pair cables
Local area networks (LANs)
Multiplexers (bandwidth managers)
Assessment of existing copper cables

Public network provided services
Switched telephone lines

125
126

4.9.1
4.9.2
4.9.3

126
126
128

General
Technical details
DC pulses


viii Contents

4.9.4

4.10

4.11

4.12

4.13

4.14
4.15

5
5.1
5.2

5.3

5.4

Dual tone multifrequency — DTMF

128

Analog tie lines

128

4.10.1
4.10.2
4.10.3
4.10.4
4.10.5

128
129
130
131
131

Introduction
Four wire E&M tie lines
Two wire signaling tie line
Four wire direct tie lines
Two wire direct tie lines

Analog data services

131

4.11.1
4.11.2
4.11.3
4.11.4
4.11.5
4.11.6
4.11.7

132
132
132
133
134
134
135

Introduction
Point-to-point configuration
Point-to-multipoint
Digital multipoint
Switched network DATEL service
Dedicated line DATEL service
Additional information

Digital data services

135

4.12.1
4.12.2

135
135

General
Service details

Packet switched services

136

4.13.1
4.13.2
4.13.3
4.13.4
4.13.5

136
138
138
139
139

Introduction
X.25 service
X.28 services
X.32 services
Frame relay

ISDN
ATM

139
141

Local area network systems

142

Introduction
Network topologies

142
143

5.2.1
5.2.2
5.2.3
5.2.4
5.2.5

143
144
144
144
145

Bus topology
Bus topology advantages
Bus topology disadvantages
Star topology
Ring topology

Media access methods

146

5.3.1

5.3.2

146
147

Contention systems
Token passing

IEEE 802.3 Ethernet

147

5.4.1
5.4.2
5.4.3
5.4.4
5.4.5

148
148
150
151
153

Ethernet types
10Base5 systems
10Base2 systems
10BaseT
10BaseF

Contents ix

5.4.6
5.4.7
5.4.8

5.5
5.6
5.7

5.8

5.9

5.10

5.11

5.12

10Broad36
1Base5
Collisions

153
153
153

MAC frame format

High-speed Ethernet systems

154
155

5.6.1

155

Cabling limitations

100Base-T (100Base-TX, T4, FX, T2)

156

5.7.1
5.7.2
5.7.3
5.7.4
5.7.5
5.7.6

156
157
157
158
158
159

Fast Ethernet overview

100Base-TX and FX
100BASE-T4
100Base-T2
100Base-T hubs
100Base-T adapters

Fast Ethernet design considerations

159

5.8.1
5.8.2
5.8.3

159
159
160

UTP Cabling distances 100Base-TX/T4
Fiber optic cable distances 100Base-FX
100Base-T repeater rules

Gigabit Ethernet 1000Base-T

160

5.9.1
5.9.2
5.9.3
5.9.4

5.9.5
5.9.6
5.9.7

160
161
162
163
163
163
163

Gigabit Ethernet summary
Gigabit Ethernet MAC layer
1000Base-SX for horizontal fiber
1000Base-LX for vertical backbone cabling
1000Base-CX for copper cabling
1000Base-T for category 5 UTP
Gigabit Ethernet full-duplex repeaters

Network interconnection components

164

5.10.1
5.10.2
5.10.3
5.10.4
5.10.5
5.10.6

164
165
165
166
166
167

Repeaters
Bridges
Router
Gateways
Hubs
Switches

TCP/IP protocols

169

5.11.1
5.11.2
5.11.3

170
170
171

The TCP/IP protocol structure
Routing in an Internet
Transmission control protocol (TCP)

SCADA and the Internet

172

5.12.1
5.12.2
5.12.3
5.12.4
5.12.5

173
173
174
175
175

Use of the Internet for SCADA systems
Thin client solutions
Security concerns
Other issues
Conclusion


x Contents

6
6.1
6.2

6.3

6.4
6.5

6.6

6.7
6.8
6.9
6.10

6.11

7
7.1
7.2

Modems

176

Introduction
Review of the modem

176
176

6.2.1
6.2.2

6.2.3
6.2.4
6.2.5

178
179
180
180
181

Synchronous or asynchronous
Modes of operation
Components of a modem
Modem receiver
Modem transmitter

The RS-232/RS-422/RS-485 interface standards

182

6.3.1
6.3.2
6.3.3
6.3.4
6.3.5
6.3.6
6.3.7
6.3.8
6.3.9

182
183
185
185
186
187
188
188
190

The RS-232-C interface standard for serial data communication
Electrical signal characteristics
Interface mechanical characteristics
Functional description of the interchange circuits
The sequence of asynchronous operation of the RS-232 interface
Synchronous communications
Disadvantages of the RS-232 standard
The RS-422 interface standard for serial data communications
The RS-485 interface standard for serial data communications

Flow control
Modulation techniques

191
191

6.5.1
6.5.2
6.5.3
6.5.4

6.5.5
6.5.6

192
192
192
193
194
195

Amplitude modulation (or amplitude shift keying)
Frequency modulation (or frequency shift keying — FSK)
Phase modulation (or phase shift keying (PSK))
Quadrature amplitude modulation (or QAM)
Trellis coding
DFM (direct frequency modulation)

Error detection/correction and data compression

196

6.6.1
6.6.2
6.6.3

196
197
198

MNP protocol classes

Link access protocol modem (LAP-M)
Data compression techniques

Data rate versus baud rate
Modem standards
Radio modems
Troubleshooting the system

201
202
203
207

6.10.1
6.10.2
6.10.3
6.10.4

207
208
208
209

Troubleshooting the serial link
The breakout box
Protocol analyzer
Troubleshooting the modem

Selection considerations

Central site computer facilities

210

212

Introduction
Recommended installation practice

212
212

7.2.1

212

Environmental considerations


Contents xi

7.2.2
7.2.3
7.2.4

7.3

7.4

7.5

8
8.1
8.2

8.3
8.4

9
9.1
9.2
9.3
9.4
9.5
9.6
9.7

Earthing and shielding
Cabling
Power connections

213
213
214

Ergonomic requirements

215

7.3.1

7.3.2
7.3.3
7.3.4
7.3.5

215
216
216
216
217

Typical control room layout
Lighting
Sound environment
Ventilation
Colors of equipment

Design of the computer displays

217

7.4.1
7.4.2

218
219

Operator displays and graphics
Design of screens

Alarming and reporting philosophies

Troubleshooting and maintenance

220

223

Introduction
Troubleshooting the telemetry system

223
225

8.2.1
8.2.2
8.2.3
8.2.4

225
227
227
227

The RTU and component modules
The master sites
The central site
The operator station and software

Maintenance tasks

The maintenance unit system

228
230

Specification of systems

232

Introduction
Common pitfalls
Standards
Performance criteria
Testing
Documentation
Future trends in technology

232
232
233
233
233
234
234

9.7.1
9.7.2

234
235

Software based instrumentation
Future trends in SCADA systems

Appendix A

Glossary

237

Appendix B

Interface standards

258

Appendix C

CITECT practical

262

Index

273


1
Background to SCADA

1.1

Introduction and brief history of SCADA
This manual is designed to provide a thorough understanding of the fundamental concepts
and the practical issues of SCADA systems. Particular emphasis has been placed on the
practical aspects of SCADA systems with a view to the future. Formulae and details that
can be found in specialized manufacturer manuals have been purposely omitted in favor
of concepts and definitions.
This chapter provides an introduction to the fundamental principles and terminology
used in the field of SCADA. It is a summary of the main subjects to be covered
throughout the manual.
SCADA (supervisory control and data acquisition) has been around as long as there
have been control systems. The first ‘SCADA’ systems utilized data acquisition by means
of panels of meters, lights and strip chart recorders. The operator manually operating
various control knobs exercised supervisory control. These devices were and still are used
to do supervisory control and data acquisition on plants, factories and power generating
facilities. The following figure shows a sensor to panel system.

Sensors

Figure 1.1
Sensors to panel using 4–20 mA or voltage


2 Practical SCADA for Industry

The sensor to panel type of SCADA system has the following advantages:
• It is simple, no CPUs, RAM, ROM or software programming needed
• The sensors are connected directly to the meters, switches and lights on the
panel

• It could be (in most circumstances) easy and cheap to add a simple device like
a switch or indicator
The disadvantages of a direct panel to sensor system are:
• The amount of wire becomes unmanageable after the installation of hundreds
of sensors
• The quantity and type of data are minimal and rudimentary
• Installation of additional sensors becomes progressively harder as the system
grows
• Re-configuration of the system becomes extremely difficult
• Simulation using real data is not possible
• Storage of data is minimal and difficult to manage
• No off site monitoring of data or alarms
• Someone has to watch the dials and meters 24 hours a day

1.2

Fundamental principles of modern SCADA systems
In modern manufacturing and industrial processes, mining industries, public and private
utilities, leisure and security industries telemetry is often needed to connect equipment
and systems separated by large distances. This can range from a few meters to thousands
of kilometers. Telemetry is used to send commands, programs and receives monitoring
information from these remote locations.
SCADA refers to the combination of telemetry and data acquisition. SCADA
encompasses the collecting of the information, transferring it back to the central site,
carrying out any necessary analysis and control and then displaying that information on a
number of operator screens or displays. The required control actions are then conveyed
back to the process.
In the early days of data acquisition, relay logic was used to control production and
plant systems. With the advent of the CPU and other electronic devices, manufacturers
incorporated digital electronics into relay logic equipment. The PLC or programmable

logic controller is still one of the most widely used control systems in industry. As need
to monitor and control more devices in the plant grew, the PLCs were distributed and the
systems became more intelligent and smaller in size. PLCs and DCS (distributed control
systems) are used as shown below.


Background to SCADA 3

PLC
or
DCS

PC

Sensors
A fieldbus

Figure 1.2
PC to PLC or DCS with a fieldbus and sensor

The advantages of the PLC / DCS SCADA system are:
• The computer can record and store a very large amount of data
• The data can be displayed in any way the user requires
• Thousands of sensors over a wide area can be connected to the system
• The operator can incorporate real data simulations into the system
• Many types of data can be collected from the RTUs
• The data can be viewed from anywhere, not just on site
The disadvantages are:
• The system is more complicated than the sensor to panel type
• Different operating skills are required, such as system analysts and

programmer
• With thousands of sensors there is still a lot of wire to deal with
• The operator can see only as far as the PLC
As the requirement for smaller and smarter systems grew, sensors were designed with
the intelligence of PLCs and DCSs. These devices are known as IEDs (intelligent
electronic devices). The IEDs are connected on a fieldbus, such as Profibus, Devicenet or
Foundation Fieldbus to the PC. They include enough intelligence to acquire data,
communicate to other devices, and hold their part of the overall program. Each of these
super smart sensors can have more than one sensor on-board. Typically, an IED could
combine an analog input sensor, analog output, PID control, communication system and
program memory in one device.


4 Practical SCADA for Industry

PC
A fieldbus

IED's

Ethernet

Figure 1.3
PC to IED using a fieldbus

The advantages of the PC to IED fieldbus system are:
• Minimal wiring is needed
• The operator can see down to the sensor level
• The data received from the device can include information such as serial
numbers, model numbers, when it was installed and by whom

• All devices are plug and play, so installation and replacement is easy
• Smaller devices means less physical space for the data acquisition system
The disadvantages of a PC to IED system are:
• More sophisticated system requires better trained employees
• Sensor prices are higher (but this is offset somewhat by the lack of PLCs)
• The IEDs rely more on the communication system

1.3

SCADA hardware
A SCADA system consists of a number of remote terminal units (RTUs) collecting field
data and sending that data back to a master station, via a communication system. The
master station displays the acquired data and allows the operator to perform remote
control tasks.
The accurate and timely data allows for optimization of the plant operation and
process. Other benefits include more efficient, reliable and most importantly, safer
operations. This results in a lower cost of operation compared to earlier non-automated
systems.
On a more complex SCADA system there are essentially five levels or hierarchies:
• Field level instrumentation and control devices
• Marshalling terminals and RTUs
• Communications system
• The master station(s)
• The commercial data processing department computer system


Background to SCADA 5

The RTU provides an interface to the field analog and digital sensors situated at each
remote site.

The communications system provides the pathway for communication between the
master station and the remote sites. This communication system can be wire, fiber optic,
radio, telephone line, microwave and possibly even satellite. Specific protocols and error
detection philosophies are used for efficient and optimum transfer of data.
The master station (or sub-masters) gather data from the various RTUs and generally
provide an operator interface for display of information and control of the remote sites. In
large telemetry systems, sub-master sites gather information from remote sites and act as
a relay back to the control master station.

1.4

SCADA software
SCADA software can be divided into two types, proprietary or open. Companies develop
proprietary software to communicate to their hardware. These systems are sold as ‘turn
key’ solutions. The main problem with this system is the overwhelming reliance on the
supplier of the system. Open software systems have gained popularity because of the
interoperability they bring to the system. Interoperability is the ability to mix different
manufacturers’ equipment on the same system.
Citect and WonderWare are just two of the open software packages available in the
market for SCADA systems. Some packages are now including asset management
integrated within the SCADA system. The typical components of a SCADA system are
indicated in the next diagram.
Display
Server #1

Display
Server #2

PC

Printer

PC

I/O
Database
PC

RS-232

Radio
Modem

Trend Server Task
Report Server Task
Input / Output Server Task

Radio
Modem

Instrumentation
& Control

Figure 1.4
Typical SCADA system

Key features of SCADA software are:
• User interface
• Graphics displays
• Alarms

• Trends
• RTU (and PLC) interface
• Scalability

In Out

In Out

Analog

Digital


6 Practical SCADA for Industry

•
•
•
•
•

1.5

Access to data
Database
Networking
Fault tolerance and redundancy
Client/server distributed processing

Landlines for SCADA

Even with the reduced amount of wire when using a PC to IED system, there is usually a
lot of wire in the typical SCADA system. This wire brings its own problems, with the
main problem being electrical noise and interference.
Interference and noise are important factors to consider when designing and installing a
data communication system, with particular considerations required to avoid electrical
interference. Noise can be defined as the random generated undesired signal that corrupts
(or interferes with) the original (or desired) signal. This noise can get into the cable or
wire in many ways. It is up to the designer to develop a system that will have a minimum
of noise from the beginning. Because SCADA systems typically use small voltage they
are inherently susceptible to noise.
The use of twisted pair shielded cat5 wire is a requirement on most systems. Using
good wire coupled with correct installation techniques ensures the system will be as noise
free as possible.
Fiber optic cable is gaining popularity because of its noise immunity. At the moment
most installations use glass fibers, but in some industrial areas plastic fibers are
increasingly used.

Multimode
Sheath
Cladding
Light Rays
Core

Monomode

Light Ray

Figure 1.5
Glass fiber optic cables

Future data communications will be divided up between radio, fiber optic and some
infrared systems. Wire will be relegated to supplying power and as power requirements of
electronics become minimal, even the need for power will be reduced.


Background to SCADA 7

1.6

SCADA and local area networks
Local area networks (LAN) are all about sharing information and resources. To enable all
the nodes on the SCADA network to share information, they must be connected by some
transmission medium. The method of connection is known as the network topology.
Nodes need to share this transmission medium in such a way as to allow all nodes access
to the medium without disrupting an established sender.
A LAN is a communication path between computers, file-servers, terminals,
workstations, and various other intelligent peripheral equipments, which are generally
referred to as devices or hosts. A LAN allows access for devices to be shared by several
users, with full connectivity between all stations on the network. A LAN is usually owned
and administered by a private owner and is located within a localized group of buildings.
Ethernet is the most widely use LAN today because it is cheap and easy to use.
Connection of the SCADA network to the LAN allows anyone within the company with
the right software and permission, to access the system. Since the data is held in a
database, the user can be limited to reading the information. Security issues are obviously
a concern, but can be addressed.
Page Request
Text/Graphics

Display on client

Applet

Executes on client

Server

Client

Figure 1.6
Ethernet used to transfer data on a SCADA system

1.7

Modem use in SCADA systems
RTU

PC
Modem

Modem

Figure 1.7
PC to RTU using a modem

Often in SCADA systems the RTU (remote terminal unit (PLC, DCS or IED)) is located
at a remote location. This distance can vary from tens of meters to thousands of
kilometers. One of the most cost-effective ways of communicating with the RTU over
long distances can be by dialup telephone connection. With this system the devices
needed are a PC, two dialup modems and the RTU (assuming that the RTU has a built in
COM port). The modems are put in the auto-answer mode and the RTU can dial into the

PC or the PC can dial the RTU. The software to do this is readily available from RTU
manufacturers. The modems can be bought off the shelf at the local computer store.


8 Practical SCADA for Industry

Line modems are used to connect RTUs to a network over a pair of wires. These
systems are usually fairly short (up to 1 kilometer) and use FSK (frequency shift keying)
to communicate. Line modems are used to communicate to RTUs when RS-232 or RS485 communication systems are not practical. The bit rates used in this type of system are
usually slow, 1200 to 9600 bps.

1.8

Computer sites and troubleshooting
Computers and RTUs usually run without problems for a long time if left to themselves.
Maintenance tasks could include daily, weekly, monthly or annual checks. When
maintenance is necessary, the technician or engineer may need to check the following
equipment on a regular basis:
• The RTU and component modules
• Analog input modules
• Digital input module
• Interface from RTU to PLC (RS-232/RS-485)
• Privately owned cable
• Switched telephone line
• Analog or digital data links
• The master sites
• The central site
• The operator station and software
Two main rules that are always followed in repair and maintenance of electronic
systems are:

• If it is not broken, don’t fix it
• Do no harm
Technicians and engineers have caused more problems, than they started with, by doing
stupid things like cleaning the equipment because it was slightly dusty. Or trying to get
that one more .01 dB of power out of a radio and blown the amplifier in the process.


Background to SCADA 9

PC

PC

PC
Bridge

Radio
Transmitter/
Receiver

Power
Supply

Operator Station
(Optional)

PC

RTU Rack
RS-232

RS-232
RS-232

Radio
Transmitter/
Receiver

PLC Racks

Power
Supply

RTU Slave Address 1
Operator Station
(Optional)

PC

RTU Rack
RS-232
RS-232
RS-232

Radio
Transmitter/
Receiver

PLC Racks

Power

Supply

RTU Slave Address 2

Figure 1.8
Components that could need maintenance in a SCADA system

1.9

System implementation
When first planning and designing a SCADA system, consideration should be given to
integrating new SCADA systems into existing communication networks in order to avoid
the substantial cost of setting up new infrastructure and communications facilities. This
may be carried out through existing LANs, private telephone systems or existing radio
systems used for mobile vehicle communications. Careful engineering must be carried
out to ensure that overlaying of the SCADA system on to an existing communication
network does not degrade or interfere with the existing facilities.